[llvm] [InstCombine] Simplify and/or of icmp eq with op replacement (PR #70335)

[Nikita Popov via llvm-commits]

nikic wrote:

@mikaelholmen Thank you! Here is a reduced version:
```llvm
declare void @barrier()

define i1 @test1(i32 %x) {
  %cmp1 = icmp ne i32 %x, 0
  call void @barrier()
  %div = udiv i32 2147483647, %x
  %cmp2 = icmp ugt i32 %x, %div
  %or = or i1 %cmp1, %cmp2
  ret i1 %or
}
```

What happens is that we try to evaluate `%cmp2` with `%x==0`, in which case the division is UB and folds to poison. We then fold `or` with a poison operand, resulting in an overall `poison` result.

Another slight variant of this would be:
```
define i1 @test2(i32 %x) {
  %cmp1 = icmp ne i32 %x, 32 
  %shl = shl i32 1, %x
  %cmp2 = icmp ugt i32 %x, %shl 
  %or = or i1 %cmp1, %cmp2
  ret i1 %or
}
```

Same basic issue, but without UB involvement, just plain poison.

https://github.com/llvm/llvm-project/pull/70335